The primary goal of this mentored career development award is to provide the candidate with training and skills development needed to transition to an independent investigator. Mentored training will be obtained in the following areas: 1) surrogate and intermediate markers of cancer risk; 2) molecular and genetic epidemiology and statistical methods for population genetics; 3) biospecimen collection and use. The anticipated result of this mentored training and research program is the submission of an R01 application in year four of the award period. Mammographic density (MD) is a strong risk factor for breast cancer, yet it remains unclear the extent to which it differs between black, Hispanic and white women, how density changes over time and whether this trajectory differs by race/ethnicity, and what genes contribute to mammographic density in black and Hispanic women. We propose to examine the racial/ethnic patterns of mammographic density and change in density, explore early-life and adult predictors of MD and change in density, and investigate whether SNPs associated with percent density in white women are similarly associated among black and Hispanic women. Our study population is the Boston Mammography Cohort Study (BMCS). BMCS is a study of mammographic density among women obtaining mammograms at Brigham and Women's Hospital (BWH) in Boston, MA. Over 2500 women have been enrolled since 2007 and enrollment is ongoing. We anticipate that an additional ~500 women will be enrolled prior to the start of the study period. Women completed a questionnaire on breast cancer risk factors at enrollment and as a part of this study we will request that they complete a follow-up questionnaire to update their exposures. For each woman, we have collected, or will collect, all digital mammograms received as a patient at BWH. MD is assessed using VolparaTM software that calculates automated volumetric measures of percent density, absolute dense volume and absolute non-dense volume. We will attempt to collect saliva from all self-identified black (n=295) and Hispanic (n=340) women in the cohort. DNA extracted from collected saliva will be genotyped. We expect that the proposed area of research will provide: important insight into racial patterns of mammographic density, novel information about changes in density over time across racial/ethnic groups, and multiethnic data on novel measures of MD, and targets for future fine mapping of MD risk alleles in black and Hispanic women.